1. Field of the Invention
The invention relates to imaging systems of the type in which a reflective surface provides substantial focusing power and more particularly relates to catadioptric reducing systems that exploit a combination of reflection and refraction for focusing purposes. The invention has particular applicability to Newtonian objectives for microlithographic imaging at high numerical apertures using deep ultraviolet light. The invention also contemplates issues of polarization management for imaging systems.
2. Description of Related Art
Newtonian design forms in which focusing power is attained largely by reflection have been incorporated into microlithographic instruments to accurately project images while limiting chromatic aberrations. The chromatic advantages of Newtonian design forms over all-refractive imaging systems are particularly evident for imaging with ultraviolet light where transmissive material choices are more limited.
Accompanying a trend toward the use of shorter wavelengths of ultraviolet light, higher numerical apertures are also sought to achieve higher resolution by microlithographic instruments. The high refraction angles required of known refractive objectives contribute to a number of aberrations including chromatic aberrations that are difficult to correct with the limited material choices for transmitting deep ultraviolet light (i.e., less than 200 nanometers wavelength). Certain anisotropic properties, such as intrinsic birefringence, also become evident in materials at the shorter wavelengths further complicating refractive solutions.
Although reflective optics are largely chromatically insensitive, a number of other aberrations accompany their use including spherical aberration and field curvature. Accordingly, refractive optics have been used in combination with reflective optics, balancing the strengths and weaknesses of each other in catadioptric forms of Newtonian designs. Examples of such catadioptric forms are disclosed in co-assigned U.S. Pat. No. 5,650,877 entitled “Imaging System for Deep Ultraviolet Lithography”, which is hereby incorporated by reference.
Higher resolution requires either higher numerical apertures or shorter wavelengths for a given numerical aperture, and most beneficially, both. Each places demands on the imaging system. The shorter wavelengths (particularly 157-nanometer light) further limit material choices for transmission and amplify anisotropic characteristics that interfere with the uniform propagation of light. The higher numerical apertures require the severe bending of light rays that introduce a host of aberrations and further exacerbate material deficiencies.